Cyanocobalamin is the preferred supplemental form of vitamin B 12 as there is insufficient evidence to support the efficacy of the other forms like methylcobalamin. Michael Greger MD, How Not to Die. How Quickly Do Vitamin B12 Injections Work? Prime Aesthetics Vitamin B12 injections deliver high doses of this essential vitamin directly into your bloodstream for better absorption than oral supplements to boost energy, support red blood cell production and maintain nerve function. Vitamin B12 Injection Dose for Adults: 7 Key Facts on Dosage & Frequency Liv Hospital B12 Shots at Home: How, Where & How Often to Inject Yourself AZ IV Medics